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I don't have an API key handy, but it's not marked as deprecated/removed in the documentation, so I can't see why it wouldn't still be in there.

The dislike count is also still present in the [0] video API response.

> it's not marked as deprecated/removed

It's not marked as deprecated or removed because dislikes were never deprecated or removed, nor implied to be.

They made it private (readable only to the uploader) and marked it as private.

A plug-in using this API will only tell you the dislike count for your own videos.